A necessary procedure during medical surgery is to monitor the heart rate of the patient by means of a heart rate meter. A problem exists, however, if the patient has a pacemaker, since the pacer signal may continue to trigger the heart rate meter even if the patient has gone into cardiac arrest. Under these circumstances the heart rate meter would provide an erroneous reading and would falsely indicate the presence of a heartbeat. This problem and its associated risks have been recognized in the past, and attempts to overcome such risks have involved the filtering of the pacer signal from the patient's ECG signal, prior to applying the remainder of the ECG to the heart rate meter. Such filtration is possible when the pacer signal does not have any overshoot, since the pulse width of the pacer signal is extremely narrow as compared to the pulse width of a normal heartbeat. However, some pacemakers provide a pacer signal having an overshoot, also referred to as a "pacer tail", and the bandwidth of such a tail may be of the same order of magnitude as the entire bandwidth of the ECG complex, thus precluding the use of electronic filtering as a means of removing the pacer signal.
Accordingly, an object of the present invention is to inhibit the application of the pacer signal so that the ECG signal applied to the heart rate meter will encompass solely the heartbeat of the patient.